MEDIUM: make SO_REUSEPORT configurable

MEDIUM: make SO_REUSEPORT configurable

With Linux officially introducing SO_REUSEPORT support in 3.9 and
its mainstream adoption we have seen more people running into strange
SO_REUSEPORT related issues (a process management issue turning into
hard to diagnose problems because the kernel load-balances between the
new and an obsolete haproxy instance).

Also some people simply want the guarantee that the bind fails when
the old process is still bound.

This change makes SO_REUSEPORT configurable, introducing the command
line argument "-dR" and the noreuseport configuration directive.

A backport to 1.6 should be considered.